1. Field of the Invention
The present invention relates to a technique of exposure with light beams having different wavelengths.
2. Description of the Related Art
An exposure apparatus has conventionally been employed to manufacture semiconductor devices such as an IC and LSI, an image sensing device such as a CCD, a display device such as a liquid crystal panel, and a device such as a magnetic head. This exposure apparatus transfers a pattern image on an original such as a mask or reticle onto a substrate such as a wafer or glass plate via a projection optical system by projection exposure or scanning exposure.
In manufacturing a device having a high packing density using lithography in this way, the circuit pattern of a reticle is transferred onto a wafer, which is coated with a photosensitive agent such as a photoresist, by exposure via the projection optical system.
In recent years, along with a more rapid increase in the packing density of devices such as an IC and LSI, the micropatterning technique for semiconductor wafers is greatly advancing. Examples of a projection exposure apparatus which plays a key role in practicing this micropatterning technique are a reduction projection exposure apparatus (stepper) which exposes a wafer by the step & repeat scheme, and a reduction projection exposure apparatus (scanner) which performs exposure while synchronously scanning a reticle and wafer.
The above-described exposure apparatuses align the relative position between the reticle and the wafer using TTL (Through-The-Lens) measurement. In the TTL measurement, an alignment pattern on the reticle or on an index plate in its vicinity (to be referred to as a reticle-side pattern hereinafter), and an alignment pattern on the wafer or an index plate in its vicinity (to be referred to as a wafer-side pattern hereinafter) are measured via the projection optical system, thereby aligning their relative positions. In this manner, it is a common practice to use exposure light as measurement light in the TTL measurement, which is done via the projection optical system.
The resolution limit of the above-described exposure apparatuses is proportional to the exposure wavelength and inversely proportional to the numerical aperture of the projection optical system. To improve the resolution limit, exposure apparatuses have been developed by shortening the exposure wavelength and increasing the numerical aperture of the projection optical system. However, the depth of focus of the projection optical system is proportional to the exposure wavelength and inversely proportional to the square of the numerical aperture of the projection optical system. Accordingly, as the resolution of the exposure apparatuses increases, the depth of focus rapidly decreases.
The projection exposure apparatus involves the curvature of field of the projection optical system, the curvature of the reticle, the tilt of the reticle, and the tilt of the wafer. This makes it difficult to match the image plane of the projection optical system with the substrate surface. As a finer device pattern is formed to increase the packing density of semiconductor devices, function elements, which have conventionally been formed two-dimensionally, have come to be formed three-dimensionally. Under the circumstance, even an improvement in resolution limit can hardly hold the substrate within the depth of focus of the projection optical system.
To solve the above-described problem, a multiple image forming exposure method is available (e.g., see Japanese Patent No. 2654418). The multiple image forming exposure method transfers the reticle pattern onto the substrate by projection exposure using a plurality of wavelengths or a broadband wavelength (to be referred to as a plurality of wavelengths) as the exposure light. With this operation, the reticle pattern forms images at wavelength-specific positions in the optical axis direction using chromatic aberration, thus increasing the depth of focus.
As described above, the use of a plurality of wavelengths as the exposure light allows increasing the depth of focus by generating chromatic aberration. At the same time, since the TTL measurement generally uses the same wavelengths as those of the exposure light, a plurality of wavelengths are used as the TTL measurement light when a plurality of wavelengths are used as the exposure light, resulting in the generation of chromatic aberration. The generation of chromatic aberration in the TTL measurement gives rise to measurement errors in the optical axis direction and a direction perpendicular it. For example, when on-axis chromatic aberration is generated, a plurality of image forming planes are formed at different positions in the optical axis direction as an image of a reticle-side pattern is formed on the wafer via the projection optical system. This makes it impossible to determine an optimal image forming plane position. It is therefore difficult to specify an optimal position in the optical axis direction, that is, the focus position of the projection optical system, and satisfactorily form an image of the reticle pattern.
When magnification chromatic aberration is generated, a plurality of pattern images are formed in a direction perpendicular to the optical axis as an image of a reticle-side pattern is formed on the wafer via the projection optical system. It is therefore difficult to specify an optimal position in a direction perpendicular to the optical axis. If no optimal position in a direction perpendicular to the optical axis is specified, the overlay accuracy comes under its adverse influence.
As described above, when chromatic aberration is generated using a plurality of wavelengths as the exposure light, chromatic aberration is also generated in the TTL measurement, resulting in erroneous measurement.